Sibyl follows in the paw-steps of Wilberforce who served Edward Heath, Harold Wilson, James Callaghan and Margaret Thatcher. He was succeeded by Humphrey, a long-haired black and white stray named after Sir Humphrey Appleby, the Mandarin in the sitcom, Yes, Minister. Humphrey served Margaret Thatcher and John Major before being mysteriously "retired" in 1997 amid rumours that Cherie Blair was allergic to cats.
When No 10 announced that Humphrey had gone into retirement, photographers had to be taken to picture him posing with that day's newspapers to scotch speculation that he had been put down. Whitehall files later disclosed he had been pensioned off – with his food and vet bills paid from a Cabinet Office stipend – because he was no longer up to the job of chasing mice.